In recent years, mobile electronic devices such as mobile phones and notebook-sized personal computers have played an important role in the information society. The electronic devices need to be in a driving state over a long period of time, and high energy densities have been therefore required for secondary batteries used as driving power sources of the electronic devices.
In order to supply powers to the electronic devices and transporting systems such as vehicles, there is a need for high-performance lithium secondary batteries with reduced weights and increased energy densities. A lithium secondary battery has a configuration in which an electrolyte solution or a lithium solid electrolyte is sandwiched between a negative active material and a positive active material, the electrolyte solution being obtained by dissolving lithium salt into a non-aqueous solvent. Lithium ions are moving between the negative active material and the positive active material so that charging and discharging are performed.
While graphite has been used as the negative active material for lithium secondary batteries, it is not suitable for high-speed charging/discharging due to its large crystallite size (micron order). Carbon nanotubes are classified as a one-dimensional carbon nanostructure grown in one direction from a substrate surface by plasma CVD, for example. Further, carbon nanowalls (walls) are known, which are classified as a two-dimensional structure grown into the shape of a sheet in a vertical direction from a substrate surface (PTLs 1 to 4 and NPL 1).
A carbon nanowall (CNW) is a crystal with a relatively high crystal perfection level, formed of nano-sized graphite crystallites. Further, a carbon nanowall is a plate-shaped nanostructure formed of several to approximately 100 overlapping graphene sheets grown over a substrate surface with a graphite layer or an amorphous layer interposed in between, in a direction substantially vertical to the substrate surface. The graphene sheets are two-dimensionally extended to a total thickness of several nanometers to tens of nanometers.
While the height of a carbon nanowall increases to several hundred nm to one thousand and several hundred nm in proportion to the growth time, the thickness thereof stops increasing at approximately 40 nm. It is known that while growing carbon nanotubes (CNTs) requires a catalytic metal such as iron or cobalt on the substrate, growing carbon nanowalls does not particularly need a catalytic metal. As is further known, a carbon nanowall can be grown in a selective direction in which active species effective for the growth are falling down, when deposition is performed using a plasma CVD system under the conditions that the substrate temperature is roughly in the range of 400° C. to 500° C. and the pressure in a chamber is not higher than approximately 100 Pa.
Carbon nonowalls have received attention for its ideal structure as a negative electrode material capable of increasing high-speed charge/discharge characteristics of lithium secondary batteries (NPL 2 and PTLs 5 and 6). Carbon nanowalls are, however, disadvantageous in that the number of lithium atoms to be intercalated between layers is 1 in every 6 carbon atoms and the charging/discharging capacities theoretically have an upper limit of 372 mAh/g.
In the above circumstances, attention is currently focused on silicon, which is capable of obtaining more charging/discharging capacities than carbon negative electrode materials in theory, a silicon-based alloy, and a silicon oxide, for example. The reasons for this are that silicon can be used as a negative active material since it is alloyed with lithium, and that silicon can offer an increased battery capacity since it can incorporate more lithium than graphite can (see, for example, NPL 3 and PTLs 7 to 9).
While silicon is a material having a significantly larger capacity than carbons, silicon having been alloyed through absorption of lithium ions increases in volume approximately 4 times as much as the silicon before the absorption. Therefore, when silicon is used as the negative active material of a negative electrode, expansion and shrinkage repeatedly occur in the negative electrode in the charging/discharging cycle, and the negative active material is eventually destroyed mechanically. When silicon is used as the negative active material of a non-aqueous electrolyte secondary battery, the degradation of the negative active material due to the charging/discharging cycle is particularly notable and most of the battery capacity is lost after several sets of charging and discharging.
In order to deal with the above disadvantage, there has been developed a silicon-carbon composite nanostructured layer (NPL 4 and PTL 10) as a lithium battery negative electrode, in such a manner that a carbon nanostructured layer is formed by applying a slurry of carbon nanofibers or carbon nanotubes on a current collector conductive foil of copper, titanium, nickel, or etc. and sintering the slurry and a silicon sputtering layer with a thickness of 100 nm to 500 nm is formed on the carbon nanostructured layer. Further, another lithium battery negative electrode has been developed in which a film of nanoscale silicon particles is stacked on the surface of a carbon nanotube (PTL 11).
Furthermore, as shown in FIG. 20, a negative electrode material has been proposed (PTL 12 and NPL 5) with which a high capacity is achieved by arranging a negative active material 102 of silicon particles, a silicon coating, etc. so as to be supported by a wall in a vertical direction of a graphene sheet 101 of a carbon nanowall on a current collector substrate 100 and reducing change in volume of the negative active material 102 caused by charging and discharging in gaps among the graphene sheets 101.
PTL 12 discloses a configuration in which carbon nanowalls are formed to heights of approximately 5 μm to 20 μm by a plasma CVD system under the conditions that the respective flow rates of a carbonized source gas (C2F6) and a H2 gas are 15 sccm and 30 sccm and the entire pressure in a chamber is 100 mTorr (13.3 Pa), each of the carbon nanowalls being in the shape of a wall and extending on a copper foil in a direction substantially vertical to the copper foil, and gaps between the walls are filled with negative active material particles or the surfaces of the walls are covered with films.